This application claims benefit of priority under 35 U.S.C. xc2xa7 119 to German Patent Application Serial No. 101 28 497.7, filed on Jun. 12, 2001, German Patent Application Serial No. 101 48 267.1, filed on Sep. 28, 2001, and German Patent Application Serial No. 102 22 592.3, filed on May 22, 2002, the entire contents of which are incorporated
1. Field of the Invention
The present invention relates generally to piezoelectric drives, and more particularly to a piezo linear drive with a group of piezo actuator stacks for driving a moving member located in a guiding device. Furthermore, the present invention provides a method for operating a piezo linear drive.
2. Discussion of the Background
Multi-layer actuator stacks made of piezoceramic materials have been state of the art for many years. In piezoelectric drives, electrical energy is converted into movement by utilizing the electrostrictive effect, of one or more piezoelectric elements. In a piezoelectric drive that utilizes the longitudinal electrostrictive effect, the amount of the shift per unit length of the actuator is largely proportional to the intensity of the field that is applied. For a high degree of displacement it is necessary that a large tension be applied between the generally opposing electrodes or the distance between the electrodes must be reduced.
Monolithic multi-layer actuators are known to increase the piezoelectric effect and consist of a sintered stack of thin films of piezoceramic (for example, lead zirconate titanate with embedded metallic inner electrodes). The inner electrodes are alternately guided out of the stack and connected electrically via outer electrodes. When electric tension is applied to the outer electrodes, the piezo films expand in the field direction. This mechanical series connection of the individual piezo films accomplishes the nominal expansion of the entire piezoceramic already at low electric tension values. Such multi-layer actuators are known for example from publication DE 4036287 C2.
Furthermore, so-called micro-motors are used for moving a body, as disclosed, for example, in publication EP 633616 A2. Such motors comprise at least one piezoelectric plate, wherein electrodes are arranged on the plate surfaces. Bending forces are transmitted to a moving member with the help of a spacer which is located in the oscillation node on the piezoelectric plate.
Particularly in the case of adjusting and positioning devices, for example, with regard to optical components, movements in random degrees of freedom are frequently required, wherein an extremely high degree of positioning accuracy and high retaining forces must be achieved.
Although when using piezo actuators that operate in the d33 mode (electrical field and displacement parallel to the polarization direction) relatively large changes in length can be accomplished, it is problematic to realize large travel ranges of several hundred micrometers.
U.S. Pat. No. 6,150,750 teaches a piezoelectric linear stepping motor, wherein a configuration is used which includes actuators that represent a hybrid arrangement located on a joint substrate. Within a cylindrical stack configuration a first stack part is arranged as a longitudinal actuator and a second stack part as a shearing actuator. Monolithic ceramics with an insulation layer require a relatively high operating voltage which results in a reduced overall efficiency. According to one embodiment of this U.S. patent, a relatively fast approach to a basic position with a subsequent time-resolved fine positioning is not possible.
Accordingly, an object of the invention is to provide a piezo linear drive including a group of piezo actuator stacks which drive a moving member located in a guiding device. The piezo linear drive is configured to travel greater travel ranges within a short period of time and make possible a fine positioning with a high degree of accuracy on or near a target.
Yet, another object of the invention is to provide a drive having the necessary stiffness and which ensures great retaining forces independent of the respective position in the space.
Still yet another object of the invention is to provide a method for operating such a piezo linear drive.
These and other objects of the present invention are achieved by providing a novel piezo linear drive and a method for operating the same. The piezo linear drive includes a group of piezo actuator stacks configured to drive a moving member located in a guidance device, and located on a joint substrate in a hybrid arrangement; wherein within a stack extending from the substrate a first stack part is configured as a longitudinal actuator and a second stack part is configured as a shearing actuator, the second stack part is equipped with a wear-resistant end plate which is in at least one of a clamping contact and shearing contact with the moving member, and at least two identical motors are located adjacent to each other in order to perform alternating clamping and advancing movements during a stepping operation.
Fundamentally, the present invention provides that several special hybrid actuators made of multilayer ceramic are located on a joint substrate, wherein within one stack of such an actuator, extending from the substrate, a first stack part is designed as a longitudinal actuator and a second stack part as a shearing actuator. The latter stack part is equipped with a wear-resistant end plate, preferably made of ceramic, which is in clamping and/or shearing contact with the moving member. At least two identical actuators of this kind are arranged adjacent to each other in order to perform alternating clamping and feed movements relative to the moving member during the stepping operation.